1. Field of the Invention
The present invention relates to a radiation imaging apparatus and a radiation imaging system.
2. Description of Related Art
In recent years, there have been known radiation imaging systems using portable radiation imaging apparatuses (such as an FPD (Flat Panel Detector)) each of which includes two-dimensionally arranged radiation detection elements for accumulating electric charges corresponding to radiation emitted from a radiation source and transmitted through a subject and reads out the electric charges accumulated in the radiation detection elements to generate image data. Such radiation imaging systems require synchronization between a radiation emission period for emitting radiation in the radiation source and an electric charge accumulation period for accumulating electric charges in the radiation imaging apparatus in order to perform the radiation emission by the radiation source during the electric charge accumulation period.
However, in a case where a radiation imaging apparatus wirelessly communicates with a radiation control apparatus which controls a radiation source, due to a problem in real time property, the synchronization is possibly not achieved by performing synchronized communication for each radiation emission between the radiation control apparatus and the radiation imaging apparatus when performing dynamic imaging of emitting pulsed-radiation (pulse emission) at a predetermined time interval and obtaining a plurality of frame images.
Thus, for example, Patent document 1 (Patent Application Laid Open Publication No. 2010-81960) describes a technique of providing a time measurement section to measure time in a console as a radiation control apparatus which performs imaging instruction, further providing a time measurement section to measure time which is synchronized with the time measurement section of the console to an electronic cassette containing an FPD therein, controlling each of the time measurement sections to measure time, emitting radiation from a radiation source for a predetermined period of time from exposure start time which was determined in advance in the console, and generating image data indicating a radiation image by reading out electric charges accumulated in the FPD after the predetermined period of time elapses from the exposure start time in the electronic cassette.
However, in many cases, the portable radiation imaging apparatus is used in environment such as between a patient and abed in which heat is kept and temperature easily rises. Thus, the heat release may not be sufficiently ensured in the portable radiation imaging apparatus. On the other hand, the radiation control apparatus naturally releases sufficient heat with respect to the heat generation amount even during operation, and thus the influence of heat generation is negligible. Thus, even when the clocks are synchronized between the radiation control apparatus and the radiation imaging apparatus in advance, the radiation emission period and electric charge accumulation period may be out of synchronization (synchronization deviation may be generated) in some cases due to the influence of change in oscillator operation frequency caused by the temperature rise of the radiation imaging apparatus.
Also in a case where fluctuation is generated in the output of radiation emitted from the radiation source, the radiation emission period and the electric charge accumulation period may be out of synchronization.
In a case where the synchronization deviation is generated at dynamic imaging to generate a plurality of frame images and radiation is emitted also during a reading period after the electric charge accumulation period ends, for example, there remain electric charges corresponding to the radiation emitted in the reading period, and thus deterioration in image quality is generated in the next frame image.